The present invention relates to a displacement converter for detecting a mechanical displacement as a variation in a reactive impedance (such as an electrostatic capacitance), for converting such a variation into an electric signal, and for transmitting the electric signal to a receiving instrument.
Conventional displacement converters of the type described include a movable electrode disposed between two fixed electrodes for detecting a difference between two pressures. The movable electrode is displaceable in response to the pressure difference whereby a pair of electrostatic capacitances C.sub.1 and C.sub.2 disposed between the fixed electrodes and the movable electrode differentially vary, and the ratio (C.sub.1 -C.sub.2)/(C.sub.1 +C.sub.2) is proportional to the ratio of mechanical displacements .DELTA.d/d.
An example of such a conventional displacement converter is shown in FIG. 1 of the accompanying drawings. The displacement converter is supplied with electric current via a transmission line from an external power supply 2 connected to a load 1. To the external power supply 2 are coupled in series a field-effect transistor (FET) 3, a resistor 3 and a zener diode 5. A constant current is caused to flow through the zener diode 5 by the action of the FET 3. The FET 3 has a gate connected to the base of a bipolar transistor 6 from which drive currents are supplied to an oscillator 7, differential amplifiers 8, 9 and other elements. The oscillator includes a transformer having a primary winding 72 connected in series with a transistor 71 and a feedback winding 73, resistors 74 and 75, and capacitors 76 and 77. The primary winding 72 causes AC voltages to be induced across secondary windings 10, 11 and 12. Diodes 13, 14, 15 and 16 serve to pass currents only through the secondary winding 10 and 11 during positive half cycles of the induced AC voltages and to pass a current through the secondary winding 12 only during negative half cycles of the AC voltages. More specifically, the current I.sub.1 flowing through the secondary winding 10 passes through a closed circuit composed of a resistor 17, a resistor 19, a grounded capacitor 20, a capacitor C.sub.1, a diode 13, and the secondary winding 10. The current I.sub.2 flowing through the secondary winding 11 passes through a closed circuit composed of a resistor 18, the resistor 19, the grounded capacitor 20, a capacitor C.sub.2, a diode 14, and the secondary winding 11. Furthermore, the current which flows through the secondary winding 12 during negative half cycles passes through a closed circuit composed of the secondary winding 12, both the diodes 15 and 16, both the capacitors C.sub.1 and C.sub.2, the grounded capacitor 20, and the secondary winding 12.
The capacitors C.sub.1 and C.sub.2 have electrostatic capacitances which differentially vary in response to a mechanical displacement. In the positive half cycles, the current I.sub.1 induced in the secondary winding 10 and which flows through the resistors 17 and 19 has a magnitude dependent on the electrostatic capacitance of the capacitor C.sub.1, and the current I.sub.2 induced in the secondary winding 11 and which flows through the resistors 18 and 19 has a magnitude dependent on the electrostatic capacitance of the capacitor C.sub.1. As a result, a voltage E.sub.o which corresponds to the sum of the capacitances of the capacitors C.sub.1 and C.sub.2 appears as a voltage drop across the resistor 19: EQU E.sub.o =R.sub.o (I.sub.1 +I.sub.2), (1)
where R.sub.o is the resistance of the resistor 19. The voltage E.sub.o is compared in the differential amplifier 8 with a voltage drop (reference voltage) across a voltage dividing resistor 22 of a voltage divider composed of resistors 21 and 22. The differential amplifier 8 produces in response thereto an output which controls the amplitude of the oscillating voltage produced by the oscillator 7. Since the voltage across the voltage divider resistors 21 and 22 is maintained at a constant level by the zener diode 5, the voltage E.sub.o, the value of which varies in accordance with the sum of the capacitances of the capacitors C.sub.1 and C.sub.2, is controlled so as to be maintained at a constant reference voltage value.
A voltage E.sub.3, the value of which is determined by the difference (C.sub.1 -C.sub.2) between the capacitances of the capacitors C.sub.1 and C.sub.2, is produced as a voltage drop across the resistors 17 and 18. More specifically, a voltage E.sub.1, the value of which is determined by the capacitance of the capacitor C.sub.1, appears as a voltage drop across the resistor 17, and a voltage E.sub.2, the value of which is determined by the capacitance of the capacitor C.sub.2, appears as a voltage drop across the resistor 18. The resistances R.sub.1 and R.sub.2 of the resistors 17 and 18, respectively, are given as follows: EQU E.sub.1 =R.sub.1 I.sub.1 +E.sub.o, and (2) EQU E.sub.2 =R.sub.2 I.sub.2 +E.sub.o. (3)
These voltages E.sub.1 and E.sub.2 are applied via resistors 23 and 24 to the differential amplifier 9, from which the differential voltage E.sub.3 is produced: EQU E.sub.3 =E.sub.1 -E.sub.2 =R(I.sub.1 -I.sub.2), (4)
where R.sub.1 =R.sub.2 =R. An output transistor 25 is controlled by the output of the differential amplifier 9.
Assuming that the amplitude of the AC voltage induced in the secondary windings within the oscillator 7 is v and its frequency f, the following equations may be written: EQU I.sub.1 =f.multidot.v.multidot.C.sub.1, (5) EQU I.sub.2 =f.multidot.v.multidot.C.sub.2, (6) EQU E.sub.3 =R(I.sub.1 -I.sub.2)=R.multidot.f.multidot.v(C.sub.1 -C.sub.2), and (7) EQU E.sub.o =R.sub.o (I.sub.1 +I.sub.2)=R.sub.o .multidot.f.multidot.v.multidot.(C.sub.1 +C.sub.2). (8)
From equation (8) results the following equation: ##EQU1## Substituting equation (9) for f.multidot.v in eqaution (8), ##EQU2## Assuming that the area of the electrodes of each of the capacitors C.sub.1 and C.sub.2 is A, the distance between the electrodes is d, a change in the interelectrode spacing due to a mechanical displacement is .DELTA.d, and the dielectric constant is .epsilon., the capacitances of the capacitors C.sub.1 and C.sub.2 can be expressed as follows: ##EQU3## Substituting equation (12) for the corresponding term in equation (10), the following equation results: ##EQU4## The differential voltage E.sub.3 is proportional to the mechanical displacement .DELTA.d, and the magnitude of the output current flowing through output transistor 25 is directly dependent on the differential voltage E.sub.3. Thus, the output current I is in proportion to the mechanical displacement .DELTA.d.
The voltage drop generated across a resistor 28 by the output current I flowing therethrough is fed back through a resistor 26 to one of the inputs of the differential amplifier 9, the other input thereof being connected to a resistor 27. In the conventional displacement converter shown in FIG. 1, the voltage E.sub.o and the reference voltage (voltage drop across the resistor 22) are compared with each other by the differential amplifier 8, which produces a differential voltage based on which an oscillating voltage in the oscillator 7 is controlled to equalize the voltage E.sub.o to the reference voltage, that is, to maintain the voltage E.sub.o at a constant level. Therefore, if the voltage E.sub.o varies, then the differential voltage E.sub.3 varies in the manner expressed by equation (13), and hence the output current I changes, resulting in an error. Thus, the components involved in generating the reference voltage, that is, the FET 3, the zener diode 5, the transistor 6, the differential amplifier 8 and the resistor 22, are required to be highly stable in their characteristics. This is of course disadvantageous in that such components are costly. Also, stray or parasitic capacitances associated with the capacitors C.sub.1 and C.sub.2 have non-negligible effects on the output current I.
The present invention has been made in view of the above difficulties. It is an object of the present invention to provide a displacement converter which requires no components that require highly stable characteristics, is composed of a reduced number of parts with the number of differential amplifiers used being reduced, and is free from adverse effects due to stray or parasitic capacitances.